This invention relates generally to an improved stereolithography method and system for the production of three-dimensional objects.
In recent years, "stereolithography" systems, such as those described in U.S. Pat. No. 4,575,330 entitled "Apparatus For Production of Three-Dimensional Objects By Stereolithography" have come into use. Basically, stereolithography is a method for automatically building complex three-dimensional objects by successively solidifying a plurality of thin layers of a solidifiable fluid-like medium by exposure to appropriate stimulation. Successive layers are solidified on top of each other until all of the thin layers are created to form a whole object (objects made in this way are sometimes called "parts"). In a preferred embodiment the fluid medium is a liquid photopolymer that can be polymerized and solidified by exposure to UV radiation. Each polymerized layer is in essence a thin cross section of the desired three-dimensional part. This method of fabrication is extremely powerful for quickly reducing design ideas to physical form for making prototypes. Moreover, complex parts can be made quickly without tooling. Because the system uses a computer to generate the cross-sectional patterns, the system can be readily linked to a CAD system.
Presently preferred polymers are cured by ultraviolet (UV) light and their curing rates are fast enough using reasonably available UV light to make them practical building materials. The liquid that is not polymerized when a part is formed is still usable and remains in the vat for the creation of successive parts. An ultraviolet laser generates a small intense spot of UV which is moved across the liquid surface with galvanometer or servo mirror X-Y scanners in a predetermined pattern. The scanners are driven by computer generated vectors or the like. Precise complex patterns can be rapidly produced with this technique.
A preferred stereolithography system includes a laser scanner, a vat for containing the polymerizable liquid, and an object support platform, which is capable of being raised and lowered in the vat, and a controlling computer. The system is programmed and automatically makes a plastic part by forming one thin cross section at a time and building the desired three-dimensional object up layer by layer.
In original stereolithographic apparatuses actinic radiation was used to solidify the fluid medium. Teachings in this regard come from the previously mentioned U.S. Pat. No. 4,575,330. In this referenced patent, there are teachings regarding the fluid medium's ability to absorb actinic radiation as being an important factor in determining the ability of the fluid medium to be solidifiable to form thin layers of cohesive material. However, this referenced patent does not disclose methods, techniques, and apparatus necessary to predict desired properties of the solidified material and does not present methods and techniques for controlling such properties. Additional art which doesn't develop the aspects concerning this invention any further, is disclosed by E. V. Fudim. This art includes U.S. Pat. Nos. 4,752,498 and 4,801,477, along with an article in the Sep. 1985 edition of Mechanical Engineering, entitled "A New Method of Three-Dimensional Micromachining" and an article in the Mar. 6, 1986 edition of Machine Design, entitled "Sculpting Parts with Light". Of these references, the one of primary interest is the article entitled "Sculpting Parts with Light". In this article Fudim discusses the use of Beer's law and a coefficient of transmission. He also utilizes a few equations derived from Beer's law in his discussion. Fudim's teachings, however, suffer from the same failing as did the art before him. Fudim failed to teach the significance of considering a separate penetration depth for each wavelength or the contribution of beam profile information in making accurate predictions of cure depth and predictions of other related cure parameters. This failing may be linked to Fudim's emphasis on use of flood exposure in combination with masks. An earlier approach than either of the previous two was disclosed by A. J. Herbert in an article in the Journal of Applied Photographic Engineering, entitled "Solid Object Generation," dated August 1982, In this article Herbert describes the use of, presumably, a single wavelength laser to expose and solidify his material. Herbert doesn't teach the connection between absorption and ability to form thin layers. In this regard Herbert actually preferred the use of a liquid photopolymer that photobleached which makes it more difficult to get predictable and controllable results. Photobleaching refers to the resin's, and the partially solidified material's, inability to absorb actinic radiation as strongly when successive quantities of radiation are absorbed. For example, the byproducts of photoinitiator do not absorb as strongly as the photoinitiator itself. A described by Herbert his polymerized material completely stopped absorbing thereby exposing lower regions to the full beam intensity. Herbert describes the building of test objects in order to determine cure parameters such as cure depth and overcure necessary to adhere layers together.
Another earlier approach was disclosed by H. Kodama in an article in the Review of Scientific Instruments, entitled "Automatic Method for Fabricating a Three-Dimensional Plastic Model with Photo-Hardening Polymer", dated Nov. 1981. In this article, Kodama describes the use of actinic sources and also shows plots of cure depth versus exposure. Like Herbert, Kodama doesn't teach the connection between absorption, cohesiveness, and the ability to make thin layers.
All of these approaches suffer from the need to make test parts to determine cure depth versus exposure and other necessary characteristics each time a desired part is to be created. This is especially true when multiple wavelength sources are used. Since the first commercialization of this technology, by 3D Systems, Inc. of Valencia, Calif., there has been an increasing need in the art to develop techniques that lead to the rapid production of more accurate parts and stronger parts in a more timely manner. The lack of techniques to do these things have inhibited the further development of the technology. The techniques of the present invention address these issues, leading to parts being built with higher resolution, higher accuracy, shorter build times, greater efficiency, enhanced physical properties, and resulting in less operator intervention in the part building process.
Associated with the absorption of actinic radiation many curable materials closely follow Beer's law. This law states that the intensity (I) of radiation at some depth (d) into the material is related to the intensity at the surface of the material times the base of the natural logarithm system (e) to the negative power of the depth (d) divided by the depth of penetration of the material (Dp). In equation form this is EQU I(d)=I.sub.o /e.sup.d/Dp
The depth of penetration is inversely related to the material's ability to absorb radiation. For the photopolymers used in the art of reference, as well as for most other photopolymers, the depth of penetration depends on wavelength. The art of reference used resin and wavelength combinations that resulted in various net penetration depths. All the part building considerations of these references were based on the concept of a net depth of penetration, as opposed to a superposition of the various depths of penetration present. Additionally, however, they did not teach that the depth of penetration for various wavelengths could be utilized to obtain different properties from partially solidified polymer. Also, they did not teach that while using a particular wavelength(s) of actinic radiation that the penetration depth for a given resin could be optimized for part building using a particular layer thickness. Furthermore, it was not taught that to use multiple wavelengths effectively, that the depth of penetration for each wavelength must be considered. No prior art suggested allowing for a plurality of depths of penetraton in building layered parts by stereolithography. In this context there was a single mindedness associated with the use of depths of penetration and a material's ability to solidify, as opposed to the multifaceted view of the present invention.
The first commercial stereolithographic apparatuses employed the use of a single wavelength of actinic radiation and associated resins, with the combination of wavelength and curable resin having properties suited for use at a particular layer thickness. This approach was still, therefore, utilizing a single minded association between depth of penetration, cohesive properties and cure depth.
In summary, in the past, in a stereolithographic apparatus (SLA), the stimulating radiation and photopolymer resin combination have had a single-minded interpretation and utilization of the depth of penetration of the radiation into the curable resin. The ideal depth or depths of penetration, however, actually varies from situation to situation as disclosed in the detailed description below. Thus, it would be beneficial to be able to employ a plurality of depths of penetration in operation of an SLA, either singly or in simultaneous combination depending on the building circumstances and desired results, which is a primary objective and accomplishment of the present invention.
If all other things were equal, a large depth of penetration would always be preferred so that each layer of the part could be formed as quickly as possible for a given layer thickness. However, all other things are not equal. The above statement makes the assumption that the deeper the light penetrates the faster the required amount of stimulating radiation will reach a given point in order to solidify it and thereby produce the deepest thickness of cured material in the shortest time possible. Part of this assumption is correct, that more radiation will reach a particular point in the resin the longer the depth of penetration. However, the assumption is inaccurate beyond that point since the solidification (gel) point of a material is not based on the amount of light that reaches a unit volume but instead is based on the amount of light that is capable of producing the desired solidification reaction, therefore the amount that is absorbed in that unit volume. For a liquid photopolymer this corresponds to the absorption per unit volume of stimulating radiation by the photoreactive element in the resin (generally the photoinitiator) and the radiation/initiator's efficiency in forming polymer. For a reaction based on the melting of powder followed by the resolidification of the liquid formed thereby producing a truly solidified portion of the object, this corresponds to the net energy in a given volume at a particular time (energy input in a unit volume less energy leaving the unit volume), which is absorption and therefore is wavelength dependent. Therefore, there will exist a particular depth of penetration that will yield the minimum time to gel material to a particular depth. Any other depth of penetration (based on wavelengths, resin and other parameters used) will produce a longer time to gel. However, drawing speed is not the only criteria involved in the production of objects by these techniques. For the photopolymer case, one must also consider, for example, the strength of the gelled material formed, the amount of distortion produced, and the effects of excess exposure in a given region. Therefore, the depth of penetration would preferably be optimized by the present invention to yield the most desired net result based on a variety of conflicting factors. The dominance of these factors may vary from situation to situation. There are several such factors.
There are other problems that have arisen due to the fact that SLA have utilized depths of penetration in the previously described single-minded manner. For example, some lasers have multiple wavelengths or "lines" which require special consideration if they are to be used with maximum effectiveness in practicing high resolution stereolithography. Without this special consideration there are only limited methods of utilizing such a laser: 1) use of filters to screen out light of all wavelengths of a beam except one for use in an SLA since different lines generally have different penetration depths, 2) build special parts each time the system is to be used to determine build characteristics of the photopolymer beam combination. However, this first approach wastes laser power since some of the light is not used. Thus, it would be desirable to avoid this power waste without sacrificing SLA performance. In present SLA use, power waste is quite expensive. The second approach is necessary for determining build characteristics since a multiline laser may loose power differentially for each line, making curing with multiple lines unpredictable when considering only the overall intensity. The second approach, however is labor-and time-intensive. Newer, more powerful Argon ion lasers can be used while lasing at multiple lines and it is desirable to effectively employ such a laser in an SLA.
Curling is another problem that has developed in SLA use. This occurs because the photopolymer shrinks upon hardening. As the top layer hardens and shrinks, it tends to pull up on the previous layer causing a distortion known as curl. This is undesirable because it distorts the shape of the actual object made such that it does not match the object as designed. An SLA which minimizes this distortion is very desirable. This type of distortion is described in detail in U.S. patent application Ser. No. 339,246 filed Apr. 17, 1989.
For further details of stereolithography, reference is made to U.S. Pat. No. 4,575,330 and the following pending U.S. patent applications which are incorporated herein by this reference in their entirety, including appendices attached thereto or material incorporated therein by reference, as if fully set forth herein:
U.S. patent application Ser. No. 07/339,246. PA0 U.S. patent application Ser. No. 07/331,644. PA0 U.S. Pat. No. 5,015,424. PA0 U.S. patent application Ser. No. 07/268,429. PA0 U.S. patent application Ser. No. 07/268,428, now abandoned. PA0 U.S. patent application Ser. No. 07/268,408, now abandoned. PA0 U.S. patent application Ser. No. 07/268,816. PA0 U.S. patent application Ser. No. 07/268,907. PA0 U.S. patent application Ser. No. 07/268,837. PA0 U.S. patent application Ser. No. 07/249,399. PA0 U.S. patent application Ser. No. 07/365,444. PA0 U.S. patent application Ser. No. 07/265,039, now abandoned. PA0 U.S. patent application Ser. No. 07/269,801.